An exhaust gas containing dust (particulate matter) is exhausted from industrial combustion facilities, such as an incinerator or a power generation plant for coal burning or heavy oil burning. Additionally, an SOx gas, such as SO2 or SO3, is contained in a combustion exhaust gas. In order to remove the dust and the SOx, an exhaust gas treatment system is provided in a flue on a downstream side of a combustion facility. In the exhaust gas treatment system, an NOx removal device, an air heater, a dust collection device, a wet-type desulfurization device, and a wet-type electric dust collection machine are installed in order from an upstream side, for example, as disclosed in PTL 1. The SO3 is present in a mist state after being cooled by the wet-type desulfurization device in a flow passage of the exhaust gas treatment system.
Although the SO3 mist is as fine as about 0.1 micrometer, the SO3 mist absorbs moisture and is enlarged after passing through the wet-type desulfurization device. If the enlarged mist or dust flows into the wet-type electric dust collection machine, the surface area increases more than that before the enlargement. For this reason, the electrical charging amount of the mist increases, a space charge effect becomes great, and the discharge current of the wet-type electric dust collection machine decreases remarkably. There is a strong correlation between the removal performance of the SO3 mist and the dust and the discharge current, and if an electric current decreases, the removal performance of the SO3 mist and the dust also degrades.
Thus, in PTL 1 and PTL 2, the SO3 mist and the dust are electrically charged in advance before the gas is made to flow into a dust collection unit of the wet-type electric dust collection machine. Additionally, a method is adopted in which droplets with a greater particle diameter than that of the mist are sprayed into the gas, and in order to increase the collision probability of the SO3 mist or the dust, an electrical discharge system that alternately causes positive and negative corona discharges. The charged SO3 mist and dust are attracted to the droplets that are dielectrically polarized by an electrical field by the dust collection unit by the Coulomb force or a gradient force and are absorbed into the droplets. Since the particle diameter of the droplets is great, the droplets are easily collected even by a simple collection device using collision or an inertia force of a demister provided on a downstream side of the wet-type electric dust collection machine.